This invention relates to a bus arbitration network which is for use in an information processing system which comprises a common bus network, a plurality of bus masters, and a bus arbiter. It is to be noted throughout the instant specification that each of the bus masters may be a microcomputer, a channel device, an I/O control adapter, and the like and is connected in common to the common bus network and that the information processing system may therefore be a digital electronic computer system or a controller system of a bus structure.
In an information processing system of the type described, mutual communication is carried out between a plurality of bus masters through a common bus network. During mutual communication, the common bus network is occupied or monopolized by the bus masters which are in course of communication while the others of the bus masters are kept in a quiescent state.
With this information processing system, a request or requisition is sent from each bus master to the common bus network at the beginning of communication so as to acquire a right to use the common bus network and to monopolize the common bus network. The request may be called a common bus request while the right to use the common bus network may simply be called a right of use.
Such common bus requests often collide with one another on the common bus network. A degree of priority is assigned to the bus masters in case collision of the common bus requests happens on the common bus network. The right of use is given to a selected one of the bus masters that has a degree of priority higher than any other bus masters and that can start communication.
At any rate, common bus requests must be arbitrated in the information processing network. For this purpose, a bus arbiter is often coupled to the common bus network to process or arbitrate collision of the common bus requests and to select the bus masters.
As a rule, arbitration is carried out in consideration of a degree of emergency to decide order of acquiring the right of use when the common bus requests are issued from a plurality of the bus masters. More specifically, the right of use should be decided so that the common bus network is fairly used or occupied by each of the bus masters, provided that an emergency request or particular request can be received by the bus arbiter. In addition, the arbitration must assure availability of a bus master of a lowest degree of priority within a predetermined duration and must be completed in a short time. Otherwise, data transmission is adversely affected.
Conventional arbitration needs a complicate bus arbiter circuit and wastes a very long time in order to meet fairness of arbitration. On the other hand, the arbitration indispensably becomes unfair by the use of a simple arbiter circuit so as to shorten a time for arbitration. This results in degradation of performance in a whole information processing system.